This invention relates to an active suspension system for an automobile, and more particularly, to an active suspension system for an automobile which can maintain the vehicle-positioning control and controllability/stability at both the normal driving mode and abnormal trouble mode by continuously controlling fluid pressure of an hydraulic actuator utilizing a wheel pressure control valve and a predetermined pressure-maintaining valve.
An automotive active suspension system is a system which can actively control suspension itself, by leveling the vehicle or tuning the suspension according to the conditions of road surfaces or driving condition of an automobile utilizing a pressure source which provides force for eliminating the force of inertia applied to the vehicle body or external force being transmitted from the road surface. A typical example of such an automotive active suspension system is disclosed in U.S. Pat. No. 5,085,460, and its schematic hydraulic circuit is shown in FIG. 8.
The system has a hydraulic pump 1 as a pressure source, actuators 15 arranged at each wheel, control valve units 25 as means for controlling pressure of the actuators 15, and a pressure-maintaining unit 24 for controlling pressure.
Between a pressure accumulator 3 for absorbing pressure pulsations created by the hydraulic pump 1 which is a pressure source, and a relief valve 6 for setting the system pressure a filter 5 and a check valve 4 for preventing the filter 5 from being damaged are provided, and in the return line a hydraulic oil cooler 21 and a hydraulic oil tank 23 are provided.
The actuators 15, which are arranged corresponding with each wheel, include a sub-accumulator 13 and a damping orifice 14, and the control valve units 25 are divided into two for the front wheels and the rear wheels. There are two control valves 20 in the control valve unit 25 for the rear wheels, which control each actuator 15. There is a main pressure accumulator 11 in the supply line of the control valves 20, and there are a return pressure accumulator 16 and damping orifices 17,19 in the return line.
The pressure-maintaining unit 24 as a pressure control part includes an orifice 7 and integral flow control valve 8 for preventing the vehicular height level from suddenly being changed right after the engine starting, a safety valve 10 for preventing any sudden change of the vehicular height level and for maintaining a predetermined vehicular height level if trouble occurs in the electric circuit, and a pressure-maintaining check valve 18 and a relief valve 6. The pressure control valve 20, a proportional electronic type, which is arranged on a fluid line reaching the actuators 15, controls the posture of a vehicle by supplying or exhausting the hydraulic fluid to/from the actuators 15, for example, control outputs IF, IF from a control unit based on detecting output X of a positional acceleration sensor are transmitted to each proportional solenoid of the control valves 20, whereby the pressure control valve 20 controls the pressure of each actuator 15.
Such a hydraulic circuit as above operates as follows;
The relief valve 6,maintains the system pressure constant, and the flow control valve 8, including an ON/OFF solenoid 8S which is operated by the orifice 7 and control signal IFV, has the duty of preventing any sudden change of a vehicular height level right after the engine starting.
The pressure-maintaining check valve 18 is opened/closed to maintain a predetermined vehicular height level, by controlling pilot pressure of the fail safe valve 10 which is operated by the ON/OFF solenoid 10S which is operated by control signal IFS of the control unit 27 based on detecting signal Y of a fail detector 29.
The prevention of a sudden change of a vehicular height level, which is caused by variation of the return pressure according to the change of opening degree of the pressure-maintaining check valve 18 after the engine stops, is carried out by the damping orifice 17, the return pressure accumulator 16, and the main pressure accumulator 11. The main pressure accumulator 11 fills up the quantity of shortage of supply fluid from the hydraulic pump 1 in case that the actuators 15 urgently require much fluid, absorbs the variation of the supply pressure, and prevents the sudden change of a vehicular height level by smoothly reducing the return pressure according to leakage during constantly maintaining the return pressure by the operation of the check valve 18 after the engine stops. The return pressure accumulator 16 absorbs the pressure pulsations in the return line.
The sub-accumulator 15 and damping orifice 14 of each actuator 15, arranged to corresponding with each wheel, absorb the frequency vibrations.
However, in such a conventional hydraulic circuit, the flow control valve 8 having the ON/OFF solenoid 8S and the fail safe valve 10 having the ON/OFF solenoid 10S are very complex in their structure, and they consist of numerous parts, so that their requiring manufacturing is very difficult.
Further, because the above valves 8,10 are of a directly moving spool valve type, a big solenoid is used to directly control the spool. Accordingly, their dimensions become large. As a result, they require a large installation space.
On the other hand, the conventional system causes much energy loss because the return pressure is increased by the damping orifice 19 arranged on the return line 26R in the normal control mode.